This invention relates to increasing the electromigration resistance of lead base alloys and, more particularly, to increasing the electromigration resistance of lead/tin solders.
Metal conductors carrying a high current density such as those necessary for practical solid state microelectronic configurations are susceptible to a current-induced mass transport failure mechanism, in which removal of material from one or more locations in the current path and build-up of material at one or more other locations in the current path occurs. Such mass transport phenomenon is termed "electromigration", and results in early failure of devices, either due to an open circuit caused in the current-carrying path as a consequence of the removal of material or to a short circuit formed as a consequence of build-up of material between the current-carrying path and an adjacent conductor.
For example, in the fabrication of C-4 terminals (C-4 is a designation for controlled collapse chip connection), a wettable metal area (typically a circle about 0.1 mm (0.005 inch) in diameter, although other sizes and geometries may be employed) is evaporated onto a dielectric through holes in a previously-prepared metal mask. The wettable metal area is called the BLM (ball limiting metallurgy) and is formed of layers of Cr-Cu-Au, with a total thickness of about 1 micrometer. After the BLM is formed, Pb-Sn solder (typically about 5% Sn) is evaporated through the metal mask and the solder is then reflowed (i.e., melted) to form the ball. During reflow, tin from the solder and copper from the BLM react to form Cu.sub.3 Sn plus a small amount of Cu.sub.6 Sn.sub.5. During the initial reflow and subsequent of reflows to join to a substrate, some of the Cu.sub.3 Sn and Cu.sub.6 Sn.sub.5 spalls (i.e., flakes off) into the liquid solder where it is trapped upon solidification. The resulting Cu.sub.3 Sn+Cu.sub.6 Sn.sub.5 particle distribution in the solder due to spalling is non-uniform. As a consequence, such a non-uniform distribution of the copper/tin intermetallic compounds in the lead/tin solder can increase electromigration damage in the C.sub. 4 content.
Accordingly, it is an object of the invention to reduce the electromigration activity in a lead-base alloy.
It is another object of the invention to reduce the electromigration activity in a lead/tin solder employed in solder stripe terminations.
It is yet another object of the invention to provide a substantially uniform distribution of particles of an intermetallic compound in a lead/tin solder so as to reduce electromigration activity therein.